1. Field of the Invention
The present invention relates to an image recording material which can be used as an offset printing master. More particularly, the present invention relates to a positive type photosensitive composition for infrared lasers which can be used with regard to a so-called directly producible printing plate, which can be directly produced based on digital signals output from a computer or the like.
2. Description of the Related Art
Conventionally, as systems for producing a printing plate directly from digital computer data, there have been proposed the following: (1) a system using an electrophotographic method; (2) a photopolymerization system using a combination of exposure with an Ar laser and post-heating; (3) a system in which a silver salt-sensitive material is laminated on a photosensitive resin; (4) a system using a silver master; and (5) a system which utilizes breaking down a silicone rubber layer with a laser or with electric discharge breakdown; as well as other methods.
However, the above method (1) using an electrophotographic method has complicated image formation processes such as charging, exposure, developing and the like, and thus, the apparatus for implementing this method becomes complicated and large. In the method (2), post-heating is required, and further, handling in an illuminated room is difficult since a highly sensitive printing plate is required. In the methods (3) and (4), since a silver salt is used, there are the disadvantages that processes become complicated and cost increases. Although the method (5) has a relatively high level of performance, the problem of removing silicone remaining on a plate surface remains.
On the other hand, laser technologies have developed remarkably in recent years. In particular, solid-state and semiconductor lasers that generate rays in a range from near-infrared to infrared and are high output and compact are readily available. These lasers are very useful as an exposing light source for recording in direct production of a printing plate from digital data such as digital computer data.
A positive type photosensitive composition for infrared lasers which is used as a material for directly producible printing plates is disclosed in Japanese Patent Application Laid-Open (JP-A) No.7-285275. The invention described in this publication relates to an image recording material obtained by adding to an aqueous alkaline solution-soluble resin a substance which absorbs light and generates heat and a positive type photosensitive compound such as quinonediazide compounds and the like. The positive type photosensitive compound works in image portions as a solubility inhibitor which substantially reduces the solubility of the aqueous alkaline solution-soluble resin. In non-image portions, the positive type photosensitive compound, due to being decomposed by heat, does not exhibit a solubility inhibiting ability and can be removed with developing. Consequently, an image is formed.
Such an image recording material has a disadvantage in that a positive type photosensitive compound such as a quinonediazide compound and the like reacts under white light since this compound has a light absorption range in a visible range (350 to 500 nm); therefore handling places are restricted to places illuminated by yellow light.
As a result of the investigations of the present inventors, it has been found that a positive type image can be obtained even if quinonediazide compounds are not added to an image recording material; however, an image recording material which simply excludes quinonediazide compounds exhibits a problem in that stability of sensitivity with regard to the concentration of a developing solution, namely, developing latitude, deteriorates.